The present invention relates to a process for the transfer of data between a base station and a transponder by means of an electromagnetic carrier wave, on which data bits are modulated by means of amplitude modulation.
Such a process is known from the publication EP 473 569 B1. In this case, digital data are exchanged between a base station and a passive transponder by means of an amplitude-modulated carrier wave. The individual bits of a data item comprise a time span in which the electromagnetic field is activated and a time span in which the electromagnetic field (field gap) is deactivated, the field gap serving as a separator between two successive bits. The significance of the bits is determined from the length of time in which the electromagnetic field is activated. Moreover, with the passive system the energy for the transponder is generated from the carrier field by means of absorption modulation.
Processes for the transfer of data in the field of transponders are generally used to perform an identification operation and to read special data for this or store the data in the transponder, as well as to carry out an authentication process or anti-collision process. Thus, in applications in the automotive field, the authentication process between the base station and transponder must be concluded within a duration of about 100 ms so that the user does not notice any delay. For this, a plurality of data items must be transferred in short time spans by means of a carrier wave, in which case amplitude modulation or pulse-width modulation (PWM) in particular is used as modulation process in the field of passive transponders.
Forming the basis of the data transfer between transponder and base station is a data protocol, which amongst other things defines the number of significances or logic values per data bit, wherein, in contrast to a significance of the data bits coded by the modulation index of the amplitude, in the case of a PWM-coded carrier wave the significances of the data bits are exclusively defined by means of the duration of the modulation intervals. In the case of a PWM-coded carrier wave, the different communication conditions, e.g. the oscillation start-up and decay behavior of the respective receiver coils, are taken into consideration via the duration of the modulation intervals. Moreover, the individual modulation intervals are separated by blank or intermediate regions in order to perform a clear allocation of the significance of the data bits. If the duration of the detected modulation interval lies in a blank region in the receiver, a read error is passed back to the transmitter and the corresponding data item is repeated by the transmitter. A further type of read error occurs in particular in unfavorable receiving conditions when too short or too long a duration of the modulation interval is measured and thus a false significance of the data bits is determined. This read error may be detected by a check total.
The disadvantage of previous processes is that the transfer rate is reduced by read errors. In the case of time-critical applications, this is evident as an inconvenience particularly with the further increasing security requirements in the case of an authentication operation.